CIRCULATION OF AQUEOUS SOLUTIONS. 1023 



BELT OF WEATHERING. 



The belt of weathering comprises the belt at the surface of the earth 

 iu which the rocks are not continuously saturated with water. Its thick- 

 ness is ordinarily between and 300 meters; it is commonly less than 100 

 meters, but in the arid regions it may be exceptionally 1,000 meters. In 

 the belt of weathering the openings in rocks are upon the average more 

 numerous, and larger than in any other belt. The amount of water present 

 may vary from a very minute fraction of the amount required to fill the 

 pore spaces to saturation. Therefore there is greater variation in the 

 amount of water in the belt of weathering than in any other belt. The water 

 is derived from rainfall and from the belt of saturation. At a given place, 

 the rainfall varies greatly from time to time and there is much variation in 

 the amount of water. It is shown (see pp. 416-423) that the forces 

 which control the movement of aqueous solutions in this belt are gravity 

 and molecular attraction and that these work directly and also through 

 plant roots and mechanical movements. 



So far as ore deposits 'are concerned, the direct actions only are 

 important. Gravity steadily tends to carry the water downward. Molec- 

 ular attraction ever tends to carry the water from places of more abundance 

 to places of less abundance. The movements of the water due to these two 

 factors are very complex in the upper portion of the belt of weathering, 

 but for the middle and lower jjortions the movement of the ground water 

 is mainly controlled by gravity. Therefore in these parts of the belt the 

 water usually steadily descends. In considering ore deposits this is the 

 fundamental point in connection with the belt of weathering, as the lateral 

 movements and the ascending movements are unimportant. 



The level of ground water for a given area is not fixed, and in conse- 

 quence of its variation a certain layer near the boundary between the belt 

 of weathering and the belt of cementation, may alternately be in one belt 

 and in the other. There are various causes for this fluctuation, of which 

 changes in amount of rainfall, orogenic movements, and denudation, are 

 important. Where uplift and denudation are the rule, the belt of weathering 

 migrates steadily downward and consequently encroaches on the belt of 

 cementation. Where there is subsidence and valley filling it is not uncom- 

 mon for the belt of weathering to be encroached upon by the belt of 

 cementation. These transfers from one belt to another will be seen to be 

 of considerable importance in the genesis of ore deposits. 



